The present invention relates to plasmonic lenses and their use in imaging systems.
A plasmonic lens is a lens that directs surface plasmon polaritons (SPPs) to converge towards a single focal point. SPPs are surface waves that arise at metal-dielectric interfaces when an electromagnetic field oscillates in unison with free electrons at the surface of the metallic interface constituent.
SPPs can be generated, for instance, by interaction of laser light at a given wavelength (excitation illumination) with corrugations created on metallic surfaces. These waves are confined to the interface and have a wavelength that is much smaller than the wavelength of the excitation illumination at the same frequency. The SPP has electric field components that lie both in and out of the interface plane. The out-of-plane component inside the dielectric constituent has a much higher electromagnetic energy contents and therefore obtaining a concentrated spot of this component is desirable in various devices.
A simple example of a plasmonic lens is a series of concentric rings on a metal film. Any light that hits the film from free space at normal incidence will be coupled into SPPs (this part works like a grating coupler), and the SPPs will be heading towards the center of the circles as well as outwards, away from the center of the circles. However, the out-of-plane component in a concentric configuration will destructively interfere in the center, forming a dark focal spot.